Conventionally, for example, Patent Literature 1 discloses a technique of preventing occurrence of problems such as overcurrent and abnormal heat generation resulting from a large cross current at the time of parallel input, in a parallel-connected electricity-storage system in which one or a plurality of electricity storage elements capable of charging and discharging are serially connected to form an electricity-storage element array, and a plurality of electricity-storage element arrays are connected in parallel. In the parallel-connected electricity-storage system, a cutoff switch is provided in each electricity-storage element array, and when there is one or a plurality of electricity-storage element arrays in which a voltage difference detected by a voltage monitoring unit is within a predetermined value, only a switch corresponding to the electricity-storage element array is closed to prevent the occurrence of problems (see, for example, Patent Literature 1 mentioned below).
Patent Literature 2 discloses a secondary battery with units including unit cells and charge/discharge circuits serially connected, in which DC/DC converter operating as a charge/discharge circuit are provided for each unit cells; while discharging, an output current of the DC/DC converter connected to a unit cell having a high SOC is set to be larger than that of a unit cell having a low SOC, so that the variations in the SOC are decreased; and while charging, an output current of the DC/DC converter connected to a unit cell having a high SOC is set to be less than that of a unit cell having a low SOC, so that the variations in the SOC are decreased.